Pyrrole

Short description: Organic ring compound (C₄H₄NH)

Pyrrole  | Explicit structural formula of pyrrole, with aromaticity indicated by dashed bonds | Numbered skeletal formula of pyrrole

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| Ball-and-stick model of the pyrrole molecule | Space-filling model of the pyrrole molecule

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Names
Preferred IUPAC name

1H-Pyrrole[2]

Other names

* Azole
* Imidole[1]

Identifiers

CAS Number

* 109-97-7 Y

3D model (JSmol)

* Interactive image
* Interactive image

Beilstein Reference

| 1159
ChEBI |

* CHEBI:19203 Y

ChEMBL |

* ChEMBL16225 Y

ChemSpider |

* 7736 Y

EC Number |

* 203-724-7

Gmelin Reference

| 1705

PubChem CID

* 8027

RTECS number |

* UX9275000

UNII |

* 86S1ZD6L2C Y

UN number | 1992, 1993

InChI

* InChI=1S/C4H5N/c1-2-4-5-3-1/h1-5H Y

Key: KAESVJOAVNADME-UHFFFAOYSA-N Y

* InChI=1/C4H5N/c1-2-4-5-3-1/h1-5H

SMILES

* N1C=CC=C1

* [nH]1cccc1

Properties

Chemical formula

| C4H5N
Molar mass | 67.091 g·mol−1
Density | 0.967 g cm−3
Melting point |  −23 °C (−9 °F; 250 K)
Boiling point |  129 to 131 °C (264 to 268 °F; 402 to 404 K)
Vapor pressure | 7 mmHg at 23 °C
Acidity (pKa)  | 17.5 (for the N−H proton)
Basicity (pKb)  | 13.6 (pKa 0.4 for C.A.)

Magnetic susceptibility (χ)

| −47.6×10−6 cm3 mol−1
Viscosity | 0.001225 Pa s
Thermochemistry

Heat capacity (C)

| 1.903 J K−1 mol−1

Std enthalpy of
formation (ΔfH⦵298)

| 108.2 kJ mol−1 (gas)

Std enthalpy of
combustion (ΔcH⦵298)

| 2242 kJ mol−1
Hazards
Safety data sheet | Chemical Safety Data
NFPA 704 (fire diamond)  |

Flash point |  33.33 °C (91.99 °F; 306.48 K)

Autoignition
temperature

|  550 °C (1,022 °F; 823 K)
Explosive limits | 3.1–14.8%
Related compounds

Related compounds

| Phosphole, arsole, bismole, stibole

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Y verify (what is YN ?)
Infobox references

Pyrrole is a heterocyclic, aromatic, organic compound, a five-membered ring with the formula C

4NH.[3] It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C

4NCH
3. Porphobilinogen, a trisubstituted pyrrole, is the biosynthetic precursor to many natural products such as heme.[4]

Pyrroles are components of more complex macrocycles, including the porphyrinogens and products derived therefrom, including porphyrins of heme, the chlorins, bacteriochlorins, and chlorophylls.[5]

## Contents

* 1 Properties, structure, bonding
* 2 History
* 3 Occurrence in nature
* 4 Synthesis
* 4.1 Hantzsch pyrrole synthesis
* 4.2 Knorr pyrrole synthesis
* 4.3 Paal–Knorr pyrrole synthesis
* 4.3.1 Other methods
* 5 Biosynthesis
* 6 Reactions and reactivity
* 6.1 Reaction of pyrrole with electrophiles
* 6.1.1 Acylation
* 6.2 Reaction of deprotonated pyrrole
* 6.3 Reductions
* 6.4 Cyclization reactions
* 7 Commercial uses
* 8 Analogs and derivatives
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

## Properties, structure, bonding

Pyrrole is a colorless volatile liquid that darkens readily upon exposure to air, and is usually purified by distillation immediately before use.[6] Pyrrole has a nutty odor. Pyrrole is a 5-membered aromatic heterocycle, like furan and thiophene. Unlike furan and thiophene, it has a dipole in which the positive end lies on the side of the heteroatom, with a dipole moment of 1.58 D. In CDCl3, it has chemical shifts at 6.68 (H2, H5) and 6.22 (H3, H4). Pyrrole is an extremely weak base for an amine, with a conjugate acid pKa of −3.8. The most thermodynamically stable pyrrolium cation (C4H6N+) is formed by protonation at the 2 position. Substitution of pyrrole with alkyl substituents provides a more basic molecule—for example, tetramethylpyrrole has a conjugate acid pKa of +3.7. Pyrrole is also weakly acidic at the N–H position, with a pKa of 16.5. As a hydrogen bonding Lewis acid it is classified as a hard acid and the ECW model lists its acid parameters as EA = 1.38 and CA = 0.68.

Pyrrole has aromatic character because the lone pairs of electrons on the nitrogen atom is partially delocalized into the ring, creating a 4n + 2 aromatic system (see Hückel's rule). In terms of its aromaticity, pyrrole's is modest relative to benzene but comparable to related heterocycles thiophene and furan. The resonance energies of benzene, pyrrole, thiophene, and furan are, respectively, 152, 88, 121, and 67 kJ/mol (36, 21, 29, and 16 kcal/mol).[7] The molecule is flat.

## History

Pyrrole was first detected by F. F. Runge in 1834, as a constituent of coal tar.[8] In 1857, it was isolated from the pyrolysate of bone. Its name comes from the Greek pyrrhos (πυρρός, "reddish, fiery"), from the reaction used to detect it—the red color that it imparts to wood when moistened with hydrochloric acid.[9]

## Occurrence in nature

Heme b

Pyrrole itself is not naturally occurring, but many of its derivatives are found in a variety of cofactors and natural products. Common naturally produced molecules containing pyrroles include vitamin B12, bile pigments like bilirubin and biliverdin, and the porphyrins of heme, chlorophyll, chlorins, bacteriochlorins, and porphyrinogens.[5] Other pyrrole-containing secondary metabolites include PQQ, makaluvamine M, ryanodine, rhazinilam, lamellarin, prodigiosin, myrmicarin, and sceptrin. The syntheses of pyrrole-containing haemin, synthesized by Hans Fischer was recognized by the Nobel Prize.

Pyrrole is a constituent of tobacco smoke and may contribute to its toxic effects.[10]

## Synthesis

Pyrrole is prepared industrially by treatment of furan with ammonia in the presence of solid acid catalysts, like SiO2 and Al2O3.[9]

Synthesis of pyrrole from furan

Pyrrole can also be formed by catalytic dehydrogenation of pyrrolidine.[citation needed]

Several syntheses of the pyrrole ring have been described.[11] Three routes dominate,[12] but many other methods exist.

### Hantzsch pyrrole synthesis

Main article: Chemistry:Hantzsch pyrrole synthesis

The Hantzsch pyrrole synthesis is the reaction of β-ketoesters (1) with ammonia (or primary amines) and α-haloketones (2) to give substituted pyrroles (3).[13][14]

The Hantzsch pyrrole synthesis

### Knorr pyrrole synthesis

Main article: Chemistry:Knorr pyrrole synthesis

The Knorr pyrrole synthesis involves the reaction of an α-amino ketone or an α-amino-β-ketoester with an activated methylene compound.[15][16][17] The method involves the reaction of an α-aminoketone (1) and a compound containing a methylene group α to (bonded to the next carbon to) a carbonyl group (2).[18]

The Knorr pyrrole synthesis

### Paal–Knorr pyrrole synthesis

In the Paal–Knorr pyrrole synthesis, a 1,4-dicarbonyl compound reacts with ammonia or a primary amine to form a substituted pyrrole.[19][20]

The Paal–Knorr pyrrole synthesis

#### Other methods

Van Leusen reaction pyrroles are produced by reaction of tosylmethyl isocyanide (TosMIC) with an enone in the presence of base, in a Michael addition. A 5-endo cyclization then forms the 5-membered ring, which reacts to eliminate the tosyl group. The last step is tautomerization to the pyrrole.[citation needed]

Mechanism of the Van Leusen reaction to form pyrroles

By the Barton–Zard synthesis, an isocyanoacetate reacts with a nitroalkene in a 1,4-addition, followed by 5-endo-dig cyclization, elimination of the nitro group, and tautomerization.[21]

The starting materials in the Piloty–Robinson pyrrole synthesis, named for Gertrude and Robert Robinson and Oskar Piloty, are two equivalents of an aldehyde and hydrazine.[22][23] The product is a pyrrole with substituents at the 3 and 4 positions. The aldehyde reacts with the diamine to an intermediate di-imine (R−C=N−N=C−R). In the second step, a [3,3]-sigmatropic rearrangement takes place between. Addition of hydrochloric acid leads to ring closure and loss of ammonia to form the pyrrole. The mechanism was developed by the Robinsons.

In one modification, propionaldehyde is treated first with hydrazine and then with benzoyl chloride at high temperatures and assisted by microwave irradiation:[24]

Piloty–Robinson reaction[24]

Pyrroles bearing multiple substituents have been obtained from the reaction of münchnones and alkynes. The reaction mechanism involves 1,3-dipolar cycloaddition followed by loss of carbon dioxide by a retro-Diels–Alder process. Similar reactions can be performed using azalactones.

Synthesis of pyrroles via Diels–Alder cyclization

Pyrroles can also be prepared by silver-catalyzed cyclization of alkynes with isonitriles, where R2 is an electron-withdrawing group, and R1 is an alkane, aryl group, or ester. Examples of disubstituted alkynes have also been seen to form the desired pyrrole in considerable yield. The reaction is proposed to proceed via a silver acetylide intermediate. This method is analogous to the azide–alkyne click chemistry used to form azoles.

Synthesis of pyrrole via silver click chemistry

One synthetic route to pyrrole involves the decarboxylation of ammonium mucate, the ammonium salt of mucic acid. The salt is typically heated in a distillation setup with glycerol as a solvent.[25]

Synthesis of pyrrole from ammonium mucate

## Biosynthesis

The biosynthesis of pyrrole rings begins with aminolevulinic acid (ALA), which is synthesized from glycine and succinyl-CoA. ALA dehydratase catalyzes the condensation of two ALA molecules via a Knorr-type ring synthesis to form porphobilinogen (PBG). This later reacts to form, for example, the macrocycles heme and chlorophyll.[26]

Mechanism of biosynthesis of porphobilinogen

Proline is biosynthetically derived from the amino acid L-glutamate. Glutamate-5-semialdehyde is first formed by glutamate 5-kinase (ATP-dependent) and glutamate-5-semialdehyde dehydrogenase (which requires NADH or NADPH). This can then either spontaneously cyclize to form 1-pyrroline-5-carboxylic acid, which is reduced to proline by pyrroline-5-carboxylate reductase (using NADH or NADPH), or turned into ornithine by ornithine aminotransferase, followed by cyclisation by ornithine cyclodeaminase to form proline.[27]

Zwitterionic structure of both proline enantiomers: (S)-proline (left) and (R)-proline

Proline can be used as precursor of aromatic pyrroles in secondary natural products, as in prodigiosins.

Figure 1: Structure of Prodigiosin 1 highlighting the A, B, and C pyrrole rings

The biosynthesis of Prodigiosin[28][29] involves the convergent coupling of three pyrrole type rings (labeled A, B, and C in figure 1) from L-proline, L-serine, L-methionine, pyruvate, and 2-octenal.

Ring A is synthesized from L-proline through the nonribosomal peptide synthase (NRPS) pathway (figure 2), wherein the pyrrolidine ring of proline is oxidized twice through FAD+ to yield pyrrole ring A.

Ring A is then expanded via the polyketide synthase pathway to incorporate L-serine into ring B (figure 3). Ring A fragment is transferred from the peptidyl carrier protein (PCP) to the Acyl Carrier Protein (ACP) by a KS domain, followed by transfer to malonyl-ACP via decarboxylative Claisen condensation. This fragment is then able to react with the masked carbanion formed from the PLP mediated decarboxylation of L-serine, which cyclizes in a dehydration reaction to yield the second pyrrole ring. This intermediate is then modified by methylation (which incorporates a methyl group from L-methionine onto the alcohol at the 6 position) and oxidation of the primary alcohol to the aldehyde to yield the core A–B ring structures.

## Reactions and reactivity

Due to its aromatic character, pyrrole is difficult to hydrogenate, does not easily react as a diene in Diels–Alder reactions, and does not undergo usual olefin reactions. Its reactivity is similar to that of benzene and aniline, in that it is easy to alkylate and acylate. Under acidic conditions, pyrroles oxidize easily to polypyrrole,[30] and thus many electrophilic reagents that are used in benzene chemistry are not applicable to pyrroles. In contrast, substituted pyrroles (including protected pyrroles) have been used in a broad range of transformations.[11]

### Reaction of pyrrole with electrophiles

Pyrroles generally react with electrophiles at the α position (C2 or C5), due to the highest degree of stability of the protonated intermediate.

Error creating thumbnail: Unable to save thumbnail to destinationPyrrole electrophilic substitution

Pyrroles react easily with nitrating (e.g. HNO3/Ac2O), sulfonating (Py·SO3), and halogenating (e.g. NCS, NBS, Br2, SO2Cl2, and KI/H2O2) agents.[31] Halogenation generally provides polyhalogenated pyrroles, but monohalogenation can be performed. As is typical for electrophilic additions to pyrroles, halogenation generally occurs at the 2-position, but can also occur at the 3-position by silation of the nitrogen. This is a useful method for further functionalization of the generally less reactive 3-position.[citation needed]

#### Acylation

Acylation generally occurs at the 2-position, through the use of various methods. Acylation with acid anhydrides and acid chlorides can occur with or without a catalyst.[32] 2-Acylpyrroles are also obtained from reaction with nitriles, by the Houben–Hoesch reaction. Pyrrole aldehydes can be formed by a Vilsmeier–Haack reaction.[33]

Vilsmeier–Haack formylation of pyrrole

### Reaction of deprotonated pyrrole

The NH proton in pyrroles is moderately acidic with a pKa of 17.5.[34] Pyrrole can be deprotonated with strong bases such as butyllithium and sodium hydride.[35] The resulting alkali pyrrolide is nucleophilic. Treating this conjugate base with an electrophile such as iodomethane gives N-methylpyrrole.

N-Metalated pyrrole can react with electrophiles at the N or C positions, depending on the coordinating metal. More ionic nitrogen–metal bonds (such as with lithium, sodium, and potassium) and more solvating solvents lead to N-alkylation. Nitrophilic metals, such as MgX, lead to alkylation at C (mainly C2), due to a higher degree of coordination to the nitrogen atom. In the cases of N-substituted pyrroles, metalation of the carbons is more facile. Alkyl groups can be introduced as electrophiles, or by cross-coupling reactions.[citation needed]

Pyrrole C-metalation

Substitution at C3 can be achieved through the use of N-substituted 3-bromopyrrole, which can be synthesized by bromination of N-silylpyrrole with NBS.[citation needed]

### Reductions

Pyrroles can undergo reductions to pyrrolidines and to pyrrolines.[36] For example, Birch reduction of pyrrole esters and amides produced pyrrolines, with the regioselectivity depending on the position of the electron-withdrawing group.[citation needed]

### Cyclization reactions

Pyrroles with N-substitution can undergo cycloaddition reactions such as [4+2]-, [2+2]-, and [2+1]-cyclizations. Diels-Alder cyclizations can occur with the pyrrole acting as a diene, especially in the presence of an electron-withdrawing group on the nitrogen. Vinylpyrroles can also act as dienes.[citation needed]

Pyrrole DA

Pyrroles can react with carbenes, such as dichlorocarbene, in a [2+1]-cycloaddition. With dichlorocarbene, a dichlorocyclopropane intermediate is formed, which breaks down to form 3-chloropyridine (the Ciamician–Dennstedt rearrangement).[37][38][39]

Ciamician–Dennstedt rearrangement

## Commercial uses

Polypyrrole is of some commercial value. N-Methylpyrrole is a precursor to N-methylpyrrolecarboxylic acid, a building-block in pharmaceutical chemistry.[9] Pyrroles are also found in several drugs, including atorvastatin, ketorolac, and sunitinib. Pyrroles are used as lightfast red, scarlet, and carmine pigments.[40][41]

## Analogs and derivatives

Structural analogs of pyrrole include:

* Pyrroline, a partially saturated analog with one double bond
* Pyrrolidine, the saturated hydrogenated analog

Derivatives of pyrrole include indole, a derivative with a fused benzene ring.

## See also

* Simple aromatic rings
* Tetrapyrrole
* Polypyrrole
* Azonine

## References

1. ↑ William M. Haynes (2016). CRC Handbook of Chemistry and Physics (97th ed.). Boca Raton: CRC Press. pp. 3–478. ISBN 978-1-4987-5429-3.
2. ↑ International Union of Pure and Applied Chemistry (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. The Royal Society of Chemistry. pp. 141. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4.
3. ↑ Loudon, Marc G. (2002). "Chemistry of Naphthalene and the Aromatic Heterocycles". Organic Chemistry (4th ed.). New York: Oxford University Press. pp. 1135–1136. ISBN 978-0-19-511999-2.
4. ↑ Cox, Michael; Lehninger, Albert L.; Nelson, David R. (2000). Lehninger Principles of Biochemistry. New York: Worth Publishers. ISBN 978-1-57259-153-0. https://archive.org/details/lehningerprincip01lehn.
5. ↑ 5.0 5.1 Jusélius, Jonas; Sundholm, Dage (2000). "The aromatic pathways of porphins, chlorins and bacteriochlorins". Phys. Chem. Chem. Phys. 2 (10): 2145–2151. doi:10.1039/b000260g. Bibcode: 2000PCCP....2.2145J. https://zenodo.org/record/900813.
6. ↑ Armarego, Wilfred L. F.; Chai, Christina L. L. (2003). Purification of Laboratory Chemicals (5th ed.). Elsevier. pp. 346.
7. ↑ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 62, ISBN 978-0-471-72091-1, https://books.google.com/books?id=JDR-nZpojeEC&printsec=frontcover
8. ↑ Runge, F. F. (1834). "Ueber einige Produkte der Steinkohlendestillation". Annalen der Physik und Chemie 31 (5): 65–78. doi:10.1002/andp.18341070502. Bibcode: 1834AnP...107...65R. http://babel.hathitrust.org/cgi/pt?id=wu.89048351654;view=1up;seq=81. See especially pages 67–68, where Runge names the compound Pyrrol (fire oil) or Rothöl (red oil).
9. ↑ 9.0 9.1 9.2 Harreus, Albrecht Ludwig. "Ullmann's Encyclopedia of Industrial Chemistry". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a22_453.
10. ↑ Fowles, Jefferson; Bates, Michael; Noiton, Dominique (March 2000). "The Chemical Constituents in Cigarettes and Cigarette Smoke: Priorities for Harm Reduction". Porirua, New Zealand: New Zealand Ministry of Health. pp. 20, 49–65. http://www.moh.govt.nz/moh.nsf/pagescm/1003/$File/chemicalconstituentscigarettespriorities.pdf.
11. ↑ 11.0 11.1 Lubell, W.; Saint-Cyr, D.; Dufour-Gallant, J.; Hopewell, R.; Boutard, N.; Kassem, T.; Dörr, A.; Zelli, R. (2013). "1H-Pyrroles (Update 2013)". Science of Synthesis 2013 (1): 157–388. https://www.thieme.de/en/thieme-chemistry/sos-knowledge-updates-2013-58727.htm.
12. ↑ Gilchrist, Thomas L. (1997). Heterocyclic Chemistry (3rd ed.). Liverpool: Longman. p. 194-196.
13. ↑ Hantzsch, A. (1890). "Neue Bildungsweise von Pyrrolderivaten". Berichte der Deutschen Chemischen Gesellschaft 23: 1474–1476. doi:10.1002/cber.189002301243. https://babel.hathitrust.org/cgi/pt?id=uc1.b3481786;view=1up;seq=1484.
14. ↑ Feist, Franz (1902). "Studien in der Furan- und Pyrrol-Gruppe". Berichte der Deutschen Chemischen Gesellschaft 35 (2): 1537–1544. doi:10.1002/cber.19020350263. https://babel.hathitrust.org/cgi/pt?id=uiug.30112025693646;view=1up;seq=305.
15. ↑ Knorr, Ludwig (1884). "Synthese von Pyrrolderivaten". Berichte der Deutschen Chemischen Gesellschaft 17 (2): 1635–1642. doi:10.1002/cber.18840170220. https://babel.hathitrust.org/cgi/pt?id=uc1.b3481769;view=1up;seq=65.
16. ↑ Knorr, L. (1886). "Synthetische Versuche mit dem Acetessigester". Annalen der Chemie 236 (3): 290–332. doi:10.1002/jlac.18862360303. https://babel.hathitrust.org/cgi/pt?id=hvd.hx3kie;view=1up;seq=702.
17. ↑ Knorr, L.; Lange, H. (1902). "Ueber die Bildung von Pyrrolderivaten aus Isonitrosoketonen". Berichte der Deutschen Chemischen Gesellschaft 35 (3): 2998–3008. doi:10.1002/cber.19020350392. https://babel.hathitrust.org/cgi/pt?id=uc1.b3481898;view=1up;seq=470.
18. ↑ Corwin, Alsoph Henry (1950). "Chapter 6: The Chemistry of Pyrrole and its Derivatives". in Elderfield, Robert Cooley. Heterocyclic Compounds. 1. New York, NY: Wiley. p. 287.
19. ↑ Paal, C. (1884), "Ueber die Derivate des Acetophenonacetessigesters und des Acetonylacetessigesters", Berichte der Deutschen Chemischen Gesellschaft 17 (2): 2756–2767, doi:10.1002/cber.188401702228, https://babel.hathitrust.org/cgi/pt?id=uc1.b3481769;view=1up;seq=1188
20. ↑ Knorr, Ludwig (1884), "Synthese von Furfuranderivaten aus dem Diacetbernsteinsäureester", Berichte der Deutschen Chemischen Gesellschaft 17 (2): 2863–2870, doi:10.1002/cber.188401702254, https://babel.hathitrust.org/cgi/pt?id=uc1.b3481769;view=1up;seq=1295
21. ↑ Li, Jie Jack (2013). Heterocyclic Chemistry in Drug Discovery. New York: Wiley. ISBN 9781118354421. https://books.google.com/books?id=p_fM6CuK6xcC.
22. ↑ Piloty, Oskar (1910). "Synthese von Pyrrolderivaten: Pyrrole aus Succinylobernsteinsäureester, Pyrrole aus Azinen". Berichte der Deutschen Chemischen Gesellschaft 43 (1): 489–498. doi:10.1002/cber.19100430182. https://babel.hathitrust.org/cgi/pt?id=uc1.b3481273;view=1up;seq=499.
23. ↑ Robinson, Gertrude Maud; Robinson, Robert (1918). "LIV. A new synthesis of tetraphenylpyrrole". J. Chem. Soc. 113: 639–645. doi:10.1039/CT9181300639.
24. ↑ 24.0 24.1 Milgram, Benjamin C.; Eskildsen, Katrine; Richter, Steven M.; Scheidt, W. Robert; Scheidt, Karl A. (2007). "Microwave-Assisted Piloty–Robinson Synthesis of 3,4-Disubstituted Pyrroles" (Note). J. Org. Chem. 72 (10): 3941–3944. doi:10.1021/jo070389+. PMID 17432915.
25. ↑ Vogel (1956). Practical Organic Chemistry. p. 837. http://www.sciencemadness.org/library/books/vogel_practical_ochem_3.pdf.
26. ↑ Walsh, Christopher T.; Garneau-Tsodikova, Sylvie; Howard-Jones, Annaleise R. (2006). "Biological formation of pyrroles: Nature's logic and enzymatic machinery". Natural Product Reports 23 (4): 517–531. doi:10.1039/b605245m. PMID 16874387.
27. ↑ Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (2000). Principles of Biochemistry (3rd ed.). New York: W. H. Freeman. ISBN 1-57259-153-6. .
28. ↑ Walsh, C. T.; Garneau-Tsodikova, S.; Howard-Jones, A. R. (2006). "Biological formation of pyrroles: Nature's logic and enzymatic machinery". Nat. Prod. Rep. 23 (4): 517–531. doi:10.1039/b605245m. PMID 16874387.
29. ↑ Hu, Dennis X. (2016). "Structure, Chemical Synthesis, and Biosynthesis of Prodiginine Natural Products". Chemical Reviews 116 (14): 7818–7853. doi:10.1021/acs.chemrev.6b00024. PMID 27314508.
30. ↑ (2nd ed.). Tianjin Nankai University. 2003. ISBN 978-7-310-00620-5.
31. ↑ Römpp Lexikon Chemie. Thieme.
32. ↑ Bailey, Denis M.; Johnson, Robert E.; Albertson, Noel F. (1971). "Ethyl Pyrrole-2-Carboxylate". Organic Syntheses 51: 100. doi:10.15227/orgsyn.051.0100.
33. ↑ Silverstein, Robert M.; Ryskiewicz, Edward E.; Willard, Constance (1956). "2-Pyrrolealdehyde". Organic Syntheses 36: 74. doi:10.15227/orgsyn.036.0074.
34. ↑ Bird, C. W.; Cheeseman, G. W. H. (1984). Comprehensive Heterocyclic Chemistry. Pergamon. pp. 39–88. ISBN 978-0-08-096519-2. http://www.sciencedirect.com/science/article/pii/B9780080965192000527None , although note that Balón, M.; Carmona, M. C.; Muñoz, M. A.; Hidalgo, J. (1989). "The acid-base properties of pyrrole and its benzologs indole and carbazole: a re-examination from the excess acidity method". Tetrahedron (Great Britain: Pergamon) 45 (23): 7501–7504. doi:10.1016/S0040-4020(01)89212-7None suggests that figure's revision to 17.3.
35. ↑ Franck, Heinz-Gerhard; Stadelhofer, Jürgen Walter (1987). Industrielle Aromatenchemie: Rohstoffe, Verfahren, Produkte. Berlin: Springer. pp. 403–404. ISBN 978-3-662-07876-1.
36. ↑ Lyastukhin, Voronov [Ластухін, Воронов] (2006). pp. 781–782. ISBN 966-7022-19-6. https://archive.org/details/isbn_9667022196.
37. ↑ Ciamician, G. L.; Dennstedt, M. (1881). "Ueber die Einwirkung des Chloroforms auf die Kaliumverbindung Pyrrols". Berichte der Deutschen Chemischen Gesellschaft 14: 1153–1162. doi:10.1002/cber.188101401240. https://babel.hathitrust.org/cgi/pt?id=hvd.cl1hza;view=1up;seq=1223.
38. ↑ Corwin, Alsoph Henry (1950). Elderfield, Robert Cooley. ed. Heterocyclic Compounds. 1. New York, NY: Wiley. p. 309.
39. ↑ Mosher, H. S. (1950). Elderfield, Robert Cooley. ed. Heterocyclic Compounds. 1. New York, NY: Wiley. p. 475.
40. ↑ "DPP Pigments,Diketopyrrolopyrrole Pigments,DPP Pigments Wholesaler,Diketopyrrolopyrrole Pigments Suppliers". http://dyes-pigments.standardcon.com/dpp-pigments.html.
41. ↑ Kaur, Matinder; Choi, Dong Hoon (2015). "Diketopyrrolopyrrole: brilliant red pigment dye-based fluorescent probes and their applications". Chemical Society Reviews 44 (1): 58–77. doi:10.1039/C4CS00248B. PMID 25186723.

## Further reading

* Jones, R. Jones, ed (1990). Pyrroles. Part I. The Synthesis and the Physical and Chemical Aspects of the Pyrrole Ring. The Chemistry of Heterocyclic Compounds. 48. Chichester: John Wiley & Sons. 351. doi:10.1002/recl.19911100712. ISBN 978-0-471-62753-1.
* Jolicoeur, Benoit; Chapman, Erin E.; Thompson, Alison; Lubell, William D. (2006). "Pyrrole protection". Tetrahedron 62 (50): 11531–11563. doi:10.1016/j.tet.2006.08.071.

## External links

* Synthesis of pyrroles (overview of recent methods)
* Substitution reaction mechanisms of nitrogen-containing heteroaromatics

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